1. Field of the Invention
This invention relates to methods and apparatus for detecting and obtaining information about (particularly locating) changes in variables.
2. Introduction to the Invention
A number of methods have been used (or proposed for use) to detect changes in variables along an elongate path, e.g. the occurrence of a leak (of water or another liquid or gas), insufficient or excessive pressure, too high or too low a temperature, the presence or absence of light or another form of electro-magnetic radiation, or a change in the physical position of a movable member, e.g. a valve in a chemical process plant or a window in a building fitted with a burglar alarm system. Changes of this kind are referred to in this specification by the generic term "event". Such detection methods are for example highly desirable to detect leaks from steam lines into thermal insulation surrounding such lines, leaks from tanks and pipes containing corrosive or noxious chemicals, or leakage or condensation of water under floors or within telecommunication or electrical power systems. Some of these known methods not only signal when the event takes place, but also indicate the location of the event. However, the known methods which indicate the location of the event suffer from serious disadvantages. For example, they make use of time domain reflectometer techniques (and are, therefore, expensive), and/or give unreliable results when used over usefully long elongate paths or under conditions when there may be a substantial and unknown variation along the length of the path of a variable which effects the accuracy of the measurement (especially temperature), and/or make use of electrical conductors whose primary purpose is to carry a current (e.g. a telecommunication signal) under normal operating conditions (and which therefore have resistance and uniformity characteristics consistent with that purpose), and/or cannot be used when the event causes electrical connection between two conductors through a connection which is of high or indeterminate resistance, e.g. an ionically conductive connecting element. Reference may be made for example to U.S. Pat. Nos. 1,084,910, 2,581,213, 3,248,646, 3,384,493, 3,800,216, 3,991,413, 4,278,931 and 4,400,663, U.K. Patents Nos. 1,481,850 and 182,339 and German Offenlegungschriften Nos. 3,001,150.0 and 3,225,742. The disclosures of each of the patents, applications and other publications referred to above are incorporated herein by reference.